Angiotensin I stability (whether the peptide is iodinated or not) in buffer solutions containing protein and preservatives (i.e., sodium azide and/or ethylenediamine tetraacetic acid (EDTA)) has been a serious problem for many years. It is common practice in the art to keep angiotensin I solutions frozen at -20.degree. C prior to use since even at refrigerator temperatures the peptide can be destroyed by proteases in a few days. At room temperature the peptide will last approximately 24 hours.
Thus all presently available preparations of angiotensin I or radioiodinated angiotensin I are shipped frozen or are lyophilized. Upon arrival, when shipped frozen, the package instructions call for aliquotting and storage at -20.degree. C or storage of the entire material at -20.degree. C with a limited number of defrosting and freezing cycles.
Lyophilized material has the practical drawback of requiring reconstitution before use. Moreover, the package instructions for storage after reconstitution vary from storge at 4.degree. C for 2 months to thawing of reconstituted vials stored at -20.degree. C as needed and thereafter storage at 4.degree. C.
It has been known in the art to add PMSF to plasma samples undergoing assay for renin activity as a means for maximizing the production of angiotensin I during the assay by preventing loss of this compound through side reactions. See in this regard U.S. Pat. Nos. 3,919,407 and 3,984,352. However, the angiotensin I standards and .sup.125 I angiotensin I are indicated in these patents to be derived from a commercial kit thereby implying that they were stabilized in accordance with prior art procedures as discussed above.